The Ampere, Redefined

The ampere is a special case in the physics of electricity: Though it is a very basic dimension, and, like volt and ohm, it is one of the central units of electricity, it hitherto was not possible to measure it directly. Instead, it was necessary to take a detour over voltage and resistance to measure the current. Volt and ohm can be implemented based on natural constants -- the Josephson constant and the von-Klitzing constant.

Scientists across the globe therefore are working to find a similar constant to determine the unit of 1 ampere. A suitable natural constant could be the charge of a single electron. This charge in principle can be measured by tunneling single electrons in a suited circuit using quantum mechanics. A potential tool towards this end could be a single-electron pump, which has been known since 1990. However, it took the development of PTB researcher Hans Werner Schumacher and his team to transform the theoretical knowledge into the real world and measure the charge difference associated with every single "jump" of an electron, directly and very accurately.

Schumacher and his team developed a so-called self-referencing quantum current source -- a semiconductor circuit with multiple electron pumps and detectors. The device is operated very closely to absolute zero (0°K). In terms of topology, the single-electron pump is a tiny island with two electric connections. In pumping mode, an electron is placed on the island across one of the connections. In a second step, it is "fired" from the island across the other connection. If this process is repeated periodically, a current is generated which is determined only through the clock cycle and the charge of a single electron.

Such circuits have been said to be a promising candidate for the implementation of a physical ampere for quite a while. The merit of Schumacher and his team is that they, for the first time, succeeded in measuring the current generated at each electron jump. The electron pump devised by the PTB team transports only a few dozens electrons per second -- few enough to enable the high-precision measurements necessary to determine the real value of 1 A. The development is believed to be a decisive step towards a new definition of the ampere.

In addition, the current source from the Braunschweig, Germany, institute enables generating extremely low currents down to the attoampere range (10-18 amperes) at significantly higher accuracy than possible with conventional current measurements. This facilitates calibrating instruments for the measurement of very small currents, as they are used, for instance, in radiation protection applications.

For its achievement the Schumacher team received the €20,000 Helmholtz prize, which will be awarded on June 24 at the Helmholtz Symposium.

This article originally appeared in EE Times Europe with the headline "What is an ampere?"

Our college days have taught us that there is a difference between 'accurate' and 'precise'.

OffTopic#2 [But not way off since the following relates to both one of the 7 units of measure and NIST]:

The US Department of Commerce's National Institute of Standards and Technology (NIST) [has] launched the new atomic clock, called NIST-F2, to serve as a new US civilian time and frequency standard, along with the current NIST-F1 standard. NIST-F2 would neither gain nor lose one second in about 300 million years, making it about three times as accurate as NIST-F1, which has served as the standard since 1999, NIST said. NIST-F2 is now the world's most accurate time standard, NIST said in a statement. For now, NIST plans to simultaneously operate both NIST-F1 and NIST-F2.

"If we've learned anything in the last 60 years of building atomic clocks, we've learned that every time we build a better clock, somebody comes up with a use for it that you couldn't have foreseen," said NIST physicist Steven Jefferts, lead designer of NIST-F2.

Both clocks use a "fountain" of cesium atoms to determine the exact length of a second. Both NIST-F1 and NIST-F2 measure the frequency of a particular transition in the cesium atom - which is 9,192,631,770 vibrations per second, and is used to define the second, the international (SI) unit of time.

The reason why "kilogram" is the name of a base unit of the SI is an artefact of history.

Louis XVI charged a group of savants to develop a new system of measurement. Their work laid the foundation for the "decimal metric system", which has evolved into the modern SI. The original idea of the king's commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the "grave". By definition it would be the mass of a litre of water at the ice point (i.e. essentially1 kg). The definition was to be embodied in an artefact mass standard.

After the Revolution, the new Republican government took over the idea of the metric system but made some significant changes. For example, since many mass measurements of the time concerned masses much smaller than the kilogram, they decided that the unit of mass should be the "gramme". However, since a one-gramme standard would have been difficult to use as well as to establish, they also decided that the new definition should be embodied in a one-kilogramme artefact. This artefact became known as the "kilogram of the archives". By 1875 the unit of mass had been redefined as the "kilogram", embodied by a new artefact whose mass was essentially the same as the kilogram of the archives.

The decision of the Republican government may have been politically motivated; after all, these were the same people who condemned Lavoisier to the guillotine. In any case, we are now stuck with the infelicity of a base unit whose name has a "prefix".

OffTopic >> Thank you for the link but I find it quite odd that NIST (old NBS) calls out the unit of mass as being the Kilogram (kg) rather than a gram (g).

Wikipedia provides the following historical perspective:

In 1921 the Convention of the Metre was revised and the mandate of the CGPM (Conférence générale des poids et mesures) was extended to provide standards for all units of measure, not just mass and length. In the ensuing years the CGPM took on responsibility for providing standards of electric current (1946), luminosity (1946), temperature (1948), time (1956) and molar mass (1971). Those are not too far away in the rear view mirror!